The effect of physiotherapy on shoulder function in patients surgically treated for breast cancer was examined through a randomized controlled trial. One hundred and thirty-nine patients with newly diagnosed breast cancer were enrolled in the study. Sixty-two (45%) had Breast Conserving Therapy and 77 (55%) had Modified Radical Mastectomy (Axillary dissection of level I and II was included in both procedures). Enrolled patients were randomized to either group A or group B. Group A was offered, team instructed physiotherapy consisting of 12 sessions of 60 min, two sessions a week. The treatment was instituted between the sixth and eight postoperative week. Group B was also offered team instructed physiotherapy, consisting of 12 sessions of 60 minute two sessions a week, but not until the 26th postoperative week. The patients were seen for follow-up examinations four times during the first postoperative year (after 7, 13, 26 and 56 weeks). Shoulder function was assessed by the Constant Shoulder Score preoperatively and at the four follow-up examinations. Team instructed physiotherapy was found to improve the shoulder function significantly in patients treated surgically for breast cancer. The effect of the treatment was influenced by the type of surgery performed, and in mastectomised patients, also by the application of radiation therapy. Compromised shoulder function is a less frequent and less severe side effect to breast conserving therapy as compared to modified radical mastectomy.